@article{roh_yeo_bang_han_velikov_velev_2024, title={Transparency-changing elastomers by controlling of the refractive index of liquid inclusions}, volume={36}, ISSN={["1361-648X"]}, DOI={10.1088/1361-648X/ad6110}, abstractNote={Complex materials that change their optical properties in response to changes in environmental conditions can find applications in displays, smart windows, and optical sensors. Here a class of biphasic composites with stimuli-adaptive optical transmittance is introduced. The biphasic composites comprise aqueous droplets (a mixture of water, glycerol, and surfactant) embedded in an elastomeric matrix. The biphasic composites are tuned to be optically transparent through a careful match of the refractive indices between the aqueous droplets and the elastomeric matrix. We demonstrate that stimuli (e.g., salinity and temperature change) can trigger variations in the optical transmittance of the biphasic composite. The introduction of such transparency-changing soft matter with liquid inclusions offers a novel approach to designing advanced optical devices, optical sensors, and metamaterials.}, number={42}, journal={JOURNAL OF PHYSICS-CONDENSED MATTER}, author={Roh, Sangchul and Yeo, Seonju and Bang, Rachel S. and Han, Koohee and Velikov, Krassimir P. and Velev, Orlin D.}, year={2024}, month={Oct} }
 @article{ewins_han_bharti_robinson_velev_dimova_2022, title={Controlled adhesion, membrane pinning and vesicle transport by Janus particles}, volume={1}, ISSN={["1364-548X"]}, DOI={10.1039/d1cc07026f}, abstractNote={Iron-coated Janus particles are used to establish controlled adhesion and contact-line pinning to (artificial) cells enabling magnetic-field driven directed transport.}, journal={CHEMICAL COMMUNICATIONS}, author={Ewins, Eleanor J. and Han, Koohee and Bharti, Bhuvnesh and Robinson, Tom and Velev, Orlin D. and Dimova, Rumiana}, year={2022}, month={Jan} }
 @article{han_shields_bharti_arratia_velev_2020, title={Active Reversible Swimming of Magnetically Assembled "Microscallops" in Non-Newtonian Fluids}, volume={36}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.9b03698}, abstractNote={Miniaturized devices capable of active swimming at low Reynolds numbers are of fundamental importance and possess potential biomedical utility. The design of colloidal microswimmers requires not only miniaturizing reconfigurable structures, but also understanding their interactions with media at low Reynolds numbers. We investigate the dynamics of "microscallops" made of asymmetric magnetic cubes, which are assembled and actuated using magnetic fields. One approach to achieve directional propulsion is to break the symmetry of the viscous forces by coupling the reciprocal motions of such microswimmers with the nonlinear rheology inherent to non-Newtonian fluids. When placed in shear-thinning fluids, the local viscosity gradient resulting from non-uniform shear stresses exerted by time-asymmetric strokes of the microscallops generates propulsive thrust through an effect we term "self-viscophoresis". Surprisingly, we found that the direction of propulsion changes with the size and structure of these assemblies. We analyze the origins of their directional propulsion and explain the variable propulsion direction in terms of multiple counterbalancing domains of shear dissipation around the microscale structures. The principles governing the locomotion of these microswimmers may be extended to other reconfigurable microbots assembled from colloidal scale units.}, number={25}, journal={LANGMUIR}, author={Han, Koohee and Shields, C. Wyatt and Bharti, Bhuvnesh and Arratia, Paulo E. and Velev, Orlin D.}, year={2020}, month={Jun}, pages={7148–7154} }
 @article{ohiri_han_shields_velev_jokerst_2018, title={Propulsion and assembly of remotely powered p-type silicon microparticles}, volume={6}, ISSN={["2166-532X"]}, DOI={10.1063/1.5053862}, abstractNote={In this letter, we discuss how to prepare millions of uniform p-type silicon (Si) microparticles using top-down fabrication processes and how to remotely control their dynamics when they are suspended in water and powered by external alternating current (AC) electric fields. These microparticles present positively charged carrier types (majority carriers from boron atom doping in the intrinsic Si) and negatively charged carrier types (minority carriers from the free electrons in the Si lattice), which electrostatically affects their negatively charged surfaces and enables a variety of programmable behaviors, such as directional assembly and propulsion. At high AC electric field frequencies ( f > 10 kHz), the microparticles assemble by attractive dielectrophoretic polarization forces. At low electric field frequencies ( f ≤ 10 kHz), the microparticles propel by induced-charge electrophoretic flows. The ability to manipulate the electrostatic potential distribution within and around the microparticles (i.e., by controlling electronic carrier types through doping) is useful for designing a number of new dynamic systems and devices with precise control over their behaviors.}, number={12}, journal={APL MATERIALS}, author={Ohiri, Ugonna and Han, Koohee and Shields, C. Wyatt and Velev, Orlin D. and Jokerst, Nan M.}, year={2018}, month={Dec} }
 @article{ohiri_shields_han_tyler_velev_jokerst_2018, title={Reconfigurable engineered motile semiconductor microparticles}, volume={9}, ISSN={["2041-1723"]}, DOI={10.1038/s41467-018-04183-y}, abstractNote={AbstractLocally energized particles form the basis for emerging classes of active matter. The design of active particles has led to their controlled locomotion and assembly. The next generation of particles should demonstrate robust control over their active assembly, disassembly, and reconfiguration. Here we introduce a class of semiconductor microparticles that can be comprehensively designed (in size, shape, electric polarizability, and patterned coatings) using standard microfabrication tools. These custom silicon particles draw energy from external electric fields to actively propel, while interacting hydrodynamically, and sequentially assemble and disassemble on demand. We show that a number of electrokinetic effects, such as dielectrophoresis, induced charge electrophoresis, and diode propulsion, can selectively power the microparticle motions and interactions. The ability to achieve on-demand locomotion, tractable fluid flows, synchronized motility, and reversible assembly using engineered silicon microparticles may enable advanced applications that include remotely powered microsensors, artificial muscles, reconfigurable neural networks and computational systems.}, journal={NATURE COMMUNICATIONS}, author={Ohiri, Ugonna and Shields, C. Wyatt and Han, Koohee and Tyler, Talmage and Velev, Orlin D. and Jokerst, Nan}, year={2018}, month={May} }
 @article{shields_han_ma_miloh_yossifon_velev_2018, title={Supercolloidal Spinners: Complex Active Particles for Electrically Powered and Switchable Rotation}, volume={28}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201803465}, abstractNote={AbstractA class of supercolloidal particles that controllably spin about their central axis in AC electric fields is reported. The rational design of these “microspinners” enables their rotation in a switchable manner, which gives rise to several interesting and programmable behaviors. It is shown that due to their complex shape and discrete metallic patches on their surfaces, these microspinners convert electrical energy into active motion via the interplay of four mechanisms at different electric field frequency ranges. These mechanisms of rotation include (in order of increasing frequency): electrohydrodynamic flows, reversed electrohydrodynamic flows, induced charge electrophoresis, and self‐dielectrophoresis. As the primary mechanism powering their motion transitions from one phenomenon to the next, these microspinners display three directional spin inversions (i.e., from clockwise to anticlockwise, or vice versa). To understand the mechanisms involved, this experimental study is coupled with scaling analyses. Due to their frequency‐switchable rotation, these microspinners have potential for applications such as interlocking gears in colloidal micromachines. Moreover, the principles used to power their switchable motion can be extended to design other types of supercolloidal particles that harvest electrical energy for motion via multiple electrokinetic mechanisms.}, number={35}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Shields, Charles Wyatt and Han, Koohee and Ma, Fuduo and Miloh, Touvia and Yossifon, Gilad and Velev, Orlin D.}, year={2018}, month={Aug} }
 @article{han_shields_diwakar_bharti_lopez_velev_2017, title={Sequence-encoded colloidal origami and microbot assemblies from patchy magnetic cubes}, volume={3}, ISSN={["2375-2548"]}, DOI={10.1126/sciadv.1701108}, abstractNote={Sequence-encoded assembly of patchy magnetic microcubes enables making self-reconfiguring colloidal origami and “microbots.”}, number={8}, journal={SCIENCE ADVANCES}, author={Han, Koohee and Shields, C. Wyatt and Diwakar, Nidhi M. and Bharti, Bhuvnesh and Lopez, Gabriel P. and Velev, Orlin D.}, year={2017}, month={Aug} }
 @article{bharti_rutkowski_han_kumar_hall_velev_2016, title={Capillary Bridging as a Tool for Assembling Discrete Clusters of Patchy Particles}, volume={138}, ISSN={["0002-7863"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84996490588&partnerID=MN8TOARS}, DOI={10.1021/jacs.6b08017}, abstractNote={Janus and patchy particles are emerging as models for studying complex directed assembly patterns and as precursors of new structured materials and composites. Here we show how lipid-induced capillary bridging could serve as a new and nonconventional method of assembling patchy particles into ordered structures. Iron oxide surface patches on latex microspheres were selectively wetted with liquid lipid, driving the particle assembly into two- and three-dimensional clusters via interparticle capillary bridge formation. The liquid phase of the bridges allows local reorganization of the particles within the clusters and assists in forming true equilibrium configurations. The temperature-driven fluid-to-gel and gel-to-fluid phase transitions of the fatty acids within the bridge act as a thermal switch for cluster assembly and disassembly. By complementing the experiments with Monte Carlo simulations, we show that the equilibrium cluster morphology is determined by the patch characteristics, namely, their size, number, and shape. This study demonstrates the ability of capillary bridging as a versatile tool to assemble thermoresponsive clusters and aggregates. This method of binding particles is simple, robust, and generic and can be extended further to assemble particles with nonspherical shapes and complex surface chemistries enabling the formation of sophisticated colloidal molecules.}, number={45}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Bharti, Bhuvnesh and Rutkowski, David and Han, Koohee and Kumar, Aakash Umesh and Hall, Carol K. and Velev, Orlin D.}, year={2016}, month={Nov}, pages={14948–14953} }